target/riscv: Properly check SEW in amo_op

We're currently assuming SEW <= 3, and the "else" from
the SEW == 3 must be less.  Use a switch and explicitly
bound both SEW and SEQ for all cases.

Reviewed-by: LIU Zhiwei <zhiwei_liu@c-sky.com>
Reviewed-by: Alistair Francis <alistair.francis@wdc.com>
Signed-off-by: Richard Henderson <richard.henderson@linaro.org>
Message-id: 20211020031709.359469-8-richard.henderson@linaro.org
Signed-off-by: Alistair Francis <alistair.francis@wdc.com>